1. Field of the Invention
This invention relates to a misfire detection system for an internal combustion engine, and more particularly to such a system in which misfire detection is inactivated under specified operating conditions.
2. Description of the Prior Art
The cause for misfiring in an internal combustion engine may be related to either the fuel supply system or the ignition system of the engine. The assignee previously proposed a system for detecting misfire caused by the fuel supply system (Japanese Laid-Open Patent Publication No. 5(1993)-65866) and a system designed to improve misfire detection accuracy by prohibiting the making of misfire detections when engine combustion is apt to become unstable, as when the fuel injection system controls the air/fuel ratio lean, or when the fuel supply is being cut off (Japanese Laid-Open Patent Publication No. 5(1993)-164033).
In the prior art systems just mentioned, however, since the operation is monitored through information obtained from the engine itself, it is not possible to detect a situation in which vapor lock is liable to occur in the fuel supply system. When misfire is found to occur due to vapor lock temporarily happening, it will erroneously be judged, therefore that the structure of the engine or the fuel supply system is out of order. However, since vapor lock is an interruption of the flow of fuel in a gasoline engine caused by the formation of vapor or gas bubbles in the fuel supply system, this does not mean that the engine or the fuel supply system is degraded in its structure. On the contrary, the misfire detection can thus degrade the reliability of the system.
Thus, vapor lock is not caused by a fuel pressure drop due to trouble in the pressure regulator or fuel pump in the fuel supply system. For example, when during at a high altitude, both fuel temperature and in-fuel tank pressure tend to rise. Vapor lock often occurs if, during descent, the in-fuel tank pressure reaches the same level as the atmospheric pressure while the fuel temperature remains high.